Differences in methionine metabolism(transmethylation, remethylation & transsulfuration) may be observed in these subject groups during a primed constant infusion study using the stable isotope L-2H3-methyl-1-13C- methionine. Simultaneous infusion with labeled cysteine may give further information about transsulfuration. Studies giving the stable isotope tracers orally will help define whether TPN patients are also at risk for disturbed methionine metabolism when this amino acid is given parenterally versus enterally.